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<br />If the temporal terms in Eqs. 1 and 2 are ignored, water routing may be <br />simplified by canp.1ting water-surface profiles at successive time steps. <br />This option is available in the model. Canputation of the water-surface <br />profile at each time step is based upon the standard-step method (see Chow, <br />1957) using techniques similar to the HEC-2 computer model (1982). For many <br />cases, spatial variation in discharge due to channel storage is small and <br />this technique produces closely similar results as the unsteady routing. <br /> <br />VII. Sl!l)IMEHT ROOTIIlG <br /> <br />The sediment routing canponent for the FLUVIAL model has the following <br />major features: (1) Canp.1tation of sediment transport capacity using a <br />suitable formula for the physical conditions, (2) determination of actual <br />sediment discharge by making corrections for sorting and diffusion, (3) <br />upstream conditions for sediment inflow, and (4) numerical solution of the <br />continuity equation for sediment. These features are evaluated at each time <br />step, the results so obtained are used in determining the changes in channel <br />configuration. <br /> <br />Determination of Sediment Discharge <br /> <br />To treat the time-dependent and non-equilibrium sediment transport, <br />the bed material at each section is divided into several, say five, size <br />fractions and the size for each fraction is represented by its geanetric <br />mean. For each size fraction, sediment transport capacity is first canp.1ted <br />using a sediment transport formula. The FLUVIAL model currently provides <br />the choices of the following five sediment formulas: (1) Engelund-Hansen <br />formula (1967), (2) Yang's unit stream power formula (1972, 1986), (3) <br />Graf's formula (1970), (4) Ackers-White formula, and (5) parker, et al. <br />formula for gravel (1982). The actual sediment rate is obtained by consi- <br />dering sediment material of all size fractions already in the flow and the <br />exchange of sediment load with the bed using the method by BOrah, et al. <br />(1982) . If the stream carr ies a load in excess of its capaci ty , it wi 11 <br />deposit the excess material on the bed. In the case of erosion, any size <br />fraction available for entrainment at the bed surface will be removed by the <br />13 <br />